Once you have your files, you copy them over to %program files%\exchsvr\exchweb\themes. The folder should be called the name of the theme.

Next, let's set some registry keys:HKLM\SYSTEM\CurrentControlSet\Services\MSExchangeWeb\OWA\Themes(You'll probably have to create the "Themes" Key)

Then create a "String" value for each theme you want to create, the name of the String should be what appears in OWA.The value should be id=200;title=NAMEOFTHEME;bgcolor=#000000;path=NAMEOFFOLDER.It's that easy!